The present invention relates to a thermal barrier coating and, more particularly, to a thermal barrier coating which allows nondestructive testing to determine whether the coating has a thickness sufficient for thermal barrier effect.
A thin ceramic layer of about 0.3 to 0.5 mm in thickness is formed as a thermal barrier coating on the surface of a base member, such as a heat resistant alloy, of a hot component such as a turbine casing of a gas turbine.
A thermal barrier coating is usually formed by plasma spraying of zirconium oxide (ZrO.sub.2) stabilized by yttrium oxide (Y.sub.2 O.sub.3), calcium oxide (CaO), magnesium oxide (MgO) or a mixture thereof. When ZrO.sub.2 undergoes temperature change from a high to room temperature, phase transformation occurs, and the crystal formation changes at various portions. A difference in crystal formation leads to a difference in the volume of respective portions, and hence degradation in the coating strength is inevitable. In order to prevent this, Y.sub.2 O.sub.3, CaO, MgO or a mixture thereof is added as a solid solution to prevent phase transformation, thus increasing the strength. With this countermeasure, however, erosion of the coating due to collision of fine particles included in a high-temperature gas flow cannot be prevented.
A thermal barrier effect of a coating depends not only on its thermal conductivity but also on its thickness. More specifically, when a thermal barrier coating layer is thinner than a predetermined thickness, the thermal barrier effect is sharply decreased. It is conventionally known that a thermal barrier coating layer preferably has a thickness of more than 0.2 mm in order to provide a required thermal barrier effect.
When a thermal barrier coating is used in practice, however, it is eroded as described above, and its thickness is decreased and it cannot provide the predetermined thermal barrier effect.
Therefore, the remaining thickness of a thermal barrier coating must be detected during inspection. However, no effective nondestructive testing is available at the present. As a result, even when the current thermal barrier coating has a sufficient effect, a new thermal barrier coating is often formed, or even when the thermal barrier coating already lost its effect, it is often used without being replaced, causing great damage to the base member.